Electrodeposition of arsenic from acid electrolytes



April 17, 1956 ELECTRODEPOSITION OF ARSENIC Filed March 27, 1955 K. L- LAWRENCE EI'AL FROM ACID ELECTROLYTES 2 Sheets-Sheet l Y f 611. /L 1 q H H2504 255.0 Cu. 35.0 146 7.0 TANK 1100s: A0. 14.0 k otwsm 32.0 512 f 22 CA THODE T0 411005 FURNACE A G/z/L scr101v H2 504 Z800 I C11, 9 0 A5 6.5 Au, 14.0

\DENSITY 22.00e

EVAPORATORS 1 H2 504 5.90.0 Cu 19.0 A5 74.0 1/1; 31.0 00115111 40.0"06

55 T /v c m G/L/L I H2 504 658.0 Cu me: 5141 As 4.5 A 7 111' 31.0 M0 2.3% 7 DENSITY 44.05

NICKEL SULPHATE v CRYSTALS BLACK A010 M11 50.0% H2504 12000101 01 0.25% 011 1111.6 Fe 1.07% As 8.0 Zn 0.5 7, 7 NJ, 5.7 11 50 1.0 DEA/5m 50 06 INVENTOR. KENNETH L. LAWRENCE By 21110010{ 0001450415 0 11 PAULO/N0 0 04...... ATTO EY April 17, 1956 K. L. LAWRENCE ETAL 2,742,415

ELECTRODEPOSITION 0F ARSENIC FROM ACID ELECTROLYTES Filed March 27. 1955 2 Sheets-Sheet 2 19o I80 I70 160 ll 15o G/a/L Aczld ATTORNEY 2,742,415 J ELECTRODEPOSITION F ARSENIC FROM ACID ELECTROLYTES Kenneth L. Lawrence, Metuchen, Harold B. Croasdale, Woodbridge, and Hugh K. Spaulding,Plainfield, N; J.',

assignors to American Smelting and Refining Company,

I New York, N. Y., a corporation of New Jersey Application March 27, 1953, Serial No. 344,934

2 Claims. (Cl. 204108) This invention relates to a process for the electrodeposition of arsenic from acid electrolytes. More particularly, it relates to the removal of arsenic from copper tankhouse electrolytes by electrode position of the arsenic therefrom.

Broadly, the process comprehends establishing an acid electrolyte having more than about 4.5 grams per liter of dissolved arsenic and an acid concentration of about 550-800 grams per liter, and electrolyzing the so-established electrolyte to deposit arsenic at the cathode.

The process of the invention is based on the discovery that acid concentration is critically important in obtaining eflicient deposition of arsenic during electrolysis of acid electrolytes having arsenic dissolved therein; and that the maximum rate of arsenic deposition per amperehourisobtained with electrolytes containing acid in the range of about 550-800 grams per liter and having more than about 4.5 grams per liter of arsenic dissolved therein. 1 The invention is particularly useful in removing arsenic from ,tankhouse electrolytes used in the electrolytic refining of copper. In such copperirefining, impure copper anodes containing arsenic, along with other impurities, are electrolyzed in a suitable bath of acid-electrolyte which in commercial operations isan aqueous solution of sulfuric acid and copper sulfate. During the electrolysis the copperin the anode passes into solution-in the acid electrolyte and is deposited therefrom on the cathode as sub: stantially pure copper. Impurities associated with the copper in the anode,including arsenic, are partly dissolved in the electrolyte andin part are thrown down as anode slimes. Other impurities are substantially entirely thrown down in these slimes. p

, Because the copper anodes continuously supply .dissolved arsenic to the electrolyte, this and other impurities gradually accumulate therein and it becomes necessary either periodically or continuously to remove large quantities of the tankhouse electrolyte in order to maintain the requisite purity of the electrolytic bath. Such removed electrolyte is treated in accordance with the present process to remove arsenic therefrom, after which the electrolyte may be further treated'in any suitable manner to remove other impurities, andthe purified electrolyte returned to the tankhouse. a Thus, in a narrower aspect of the invention, there is established from the electrolyte removed from the tankhouse an electrolyte having about 550- 80( grams per liter ofsulfuric acid 'andhaving a dissolved arsenic content greater than'about 4.5 grams per liter. 'For best results, the electrolyte also preferably contains dissolved copper not in excess of about 0.9 gram per liter. Such electrolyte is then subjected to electrolysis with deposition of the arsenic at the cathode until the dissolved arsenic'is reduced to an amount not below about 4.5 grams per liter. Thereanodes cast from an anode furnace and containing arsenic,

during the electrolysis may be considered to be self adjusting'and has beenfound to assume a value in the range 'of about 1.80-2.65 'volts. Any desired amperage and current density may be used. In conducting the process on a plant-size scale, current densities of 10-20 ,amperes per square foot have been used and these'are the preferred current densities for the process. Temperature of the electrolyte appears to have no particular effect upon the electrolysis. Electrolyte temperatures up to about 130 F. may be used advantageously. However, temperatures in the range of about 80 to 130 F. are preferred.

The invention is further illustrated in the accompanying drawings and examples. It should be understood, however, that -the drawings and the examples are given for purposes of illustration and that. the invention in its broaderaspects is not limited thereto. I

. In the drawings:

Fig.'1 is a flowsheet illustrating use of the the invention electrolytes. I i

Fig. 2 illustrates arsenic removal from acid arsenical electrolytes and is a curve in which arsenic removal in terms of grams per ampere hour is plotted against acid concentration in terms of grams per liter. 2 Referring now to the flowsheet of Fig. 1, impure copper to remove arsenic from copper tankhouse together with other impurities, are placed in a sulfuric acid-copper sulfate electrolyte in the tankhouse electrolytic cells, illustrated by numeral 1. Upon passing current from theanode to the cathode, the copper in the anode dissolves in the electrolyte and is deposited as high puritycopper on the cathode during the electrolysis. The impure copper anodes also contain some oxides ofcopper which chemically dissolve in the electrolyte and.. increase its copper content. The tankhouseelectrolysis, 1 however, proceeds most efficiently when it has a copper concentration of about 40 grams per liter. The excess. copper which tends to build up in the tankhouse electrolyte due to the solution of the copper oxides, may be removedin A section cells illustrated by the numeral 2 in the 1 drawing. This may be accomplished by recycling '21 P01? tion of the tankhouse solution through the A section cells wherethe recycled liquor is electrolyzed, using aninsoluble anode. The copper removed in the cathodes in the A section cells is high grade commercial copper.

Unless removed from the electrolyte, the impurities in the tankhouse anodes which are dissolved or only partially, dissolved during the electrolysis (arsenic being included in this latter group) concentrate in the tankhouse electrolyte until they interfere with the deposition or the purity of the deposited copper at the cathode. The undissolved impurities in the anodes collect in the bottom ofthecells at the anode and are referred to in the art as anode slimes or muds. The regeneration of theelectrolyte from the cells'l by the removal of arsenic and the removal of other'impurities in the electrolyte together with after, the electrolyte, preferably after it has been further 1 i 51 arsenic-containing electrodes, may be used in the Process. ifl i sf ii, b use of the so-called Pym-Green Cells 3 For best results, when practicing the process to remove dissolved arsenic from solution and especially from tankhouse electrolytes, the anodes are insoluble anodes such f be returned to the tankhouse.

Any soluble or insoluble electrodes, includes arsenic or as, for example, lead or carbon anodes, and the cathodes are metallic cathodes, preferably'copper cathodes The voltage across the electrodes in the electrolytic cell an effective separation of the arsenic from the otherima;

purities and the copper during the regeneration may be accomplished as follows.

The concentration of the dissolved copper in the cells 1, because of the electrolysis, tends to be high in the lower portion of the cells while the concentration of other dissolved matter in the electrolyte remains uni form throughout the cells. Advantage may be taken of -:which permit continuous or partial withdrawal of part f fof the electrolyte from the upper portion of the tankhouse cells 1 thereby etfecting a small but nevertheless appreciable separation of the arsenic and other im- Patented Apr. 17, 1956 process of V a typical analysis as indicated in Fig. l, is withdrawn from these cells, preferably as a continuous stream, and may be sent to the F section cells indicated by numeral 4. In the cells 4, this withdrawn solution may be electrolyzed with an insoluble lead anode and a copper cathode to remove a major portion of the dissolved copper. Preferably, this electrolysis is continued until the copper concentration is rcduced to a value at which the copper cathodes upon continued electrolysis would become too soft, due to arsenic deposition, to be handled conveniently. Such copper cathodes from the F section cells being too impure for commercial copper, may be advantageously returned to the anode furnace for recasting for use as anodes in the copper tankhouse cells 1. i It has been found that when the copper concentration in the F section cells has been reduced in the preferred amount, crystallization of copper sulfate and other impurities does not take place or does not take place in any appreciable amount when the residual electrolyte from the F cells is subsequently concentrated by evaporation to an acid concentration of about 550- 800 grams per liter. It has been found, further, that when the copper concentration has thus been reduced to about 9 grams per liter, such crystallization and softness of the cathodes does not occur.

The electrolyte from the F section cells 4, having a typical analysis indicated in Fig. l, is passed to an evaporator or evaporators 5 where the solution is concentrated by evaporation until its acid content is increased to about 550-800 grams per liter PzSOs, the preferred value in this range being about 600 grams per liter. The thus concentrated solution contains more than about 4.5 grams per liter of dissolved arsenic and in the course of experimentation with the process on a plant-size scale over a period of several months it was found to contain not less than about 8 grams per liter of dissolved arsenic. A typical analysis of the electrolyte from the evaporator 5 is indicated in the drawing.

The electrolyte from evaporator 5 is passed to the P section cells indicated by the numeral 6 where the solution is further electrolyzed to deposit arsenic at the cathode, the electrolysis being conducted with high current efiiciency until the arsenic concentration has been reduced to a value not below about 4.5 grams per liter. As indicated by the typical analysis of the solution from the P section cells shown in the drawing, the electrolysis in these cells is continued until the arsenic concentration has been reduced to a value of about 4.5 grams per liter.

Any suitable electrodes may be used in the P section cells. The voltage of the cells is self-regulating and any desired amperage and current density may be used. In the plant-size experimentation referred to, lead anodes and copper cathodes were used. The voltage on the cells was found to be in the range of about 1.8- 2.65 volts. 5000-6000 amperes were used in the cells at a current density in the range of 1020 amperesper square foot. It was found that during this electrolysis arsenic removal wasslight until the copper was reduced to a value below about 0.9 gram per liter. The arsenic which was deposited at the cathode did not, for the most part, adhere to the cathodes but collected as a sludge at the bottom of the cell in the vicinity of the cathodes. A typical analysis of the cathode sludge is illustrated in the drawing. It was found, unless kept wet, this sludge has a strong tendency to burn spontaneously when exposed to air with the evolution of fumes of oxides of arsenic.

The sludge collected in the P section cells may, if desired, be further treated to recover valuable constituents therein. Thus, it may be washed to remove the nickel and acid therein, after which it may be burned to separate the arsenic from the copper. During the burning, the arsenic is converted to fumes of oxides of arsenic which may be cooled and collected in a baghouse. The residual copper, which is in the form of copper mixed with oxides of copper, may be used as such or further refined to pure copper.

The residual electrolyte from the P section cells may be passed to a suitable crystallizer 7, such as an iron pan crystallizer, where it may be further concentrated and cooled to precipitate nickel sulfate crystals which may have the typical analysis indicated.

The so-called black acid, which is the mother liquor from the crystallizer 7 and having the typical analysis indicated, may be used in any suitable manner for its acid values. It has been found, however, that its arsenic content and the content of the other impurities are sufiiciently low so that it may be used in the tankhouse without affecting either the efficiency of the deposition or the purity of the copper deposited at the cathodes therein. Preferably, the mother liquor is recycled to the cells 1.

' Alternately, instead of using the F section cells 4, the withdrawn tankhouse electrolyte may be passed either from the cells 3 or the cells 1 directly to the evaporator 5 and concentrated therein. In such event, copper sulfate, which may also contain nickel sulfate, may crystallize in the evaporator and may be recovered for use elsewhere. Alternatively also, the residual electrolyte from the P section cells may be returned to the tankhouse for use therein. Preferably, however, such electrolyte from the P section cells is further treated in the crystallizer 7 in the above discussed manner before it is used in the tankhouse.

The invention is further illustrated in the accompanying examples, in all of which lead anodes and copper cathodes were used.

EXAMPLE I A series of arsenical electrolytes having varying contrations of sulfuric acid were made up from black acid of the following composition:

Grams per liter H2SO4 1100 Cu 0.1 As 54.0

The test solutions were electrolyzed under the conditions noted and with the results set forth in table 1.

Table 1 Test Number 1 2 .5 4 5 6 7 81 81 81 125 120 550 660 830 550 660 27.0 32. 5 37. 8 27. 0 32. 5 0.05 0. 06 0. 07 0. 05 0. 00 .1 1.8 2.2 2.6 1.8 2.2 0. 6 0.5 0.5 4. 5 4.8 tS 2. -2. 2. 532. 2. 57 2. 5 2. Test Duration, Minutes.-- 25 o 25 35 32 360 3840 As Removed, Grams per Amp. Hour 0.114 0.136 0. 220 0.250 0.118 211 0. 245

The results obtained are plotted in the curve of Fig. 2 in which arsenic removal per ampere-hour is the ordinate and acid concentration in grams per liter is the abscissa. It will be noted that maximum current efiiciency occurs with acid concentrations in the range of about 550-800 grams per liter.

' EXAMPLE II An arsenical electrolyte having the analysis shown in Table 2 was prepared and electrolyzed, using a current of 4.6 amperes in the test cell. Arsenic removal as given in the table was obtained.

It will be noted that high copper and nickel concentrations in the electrolyte do not aifect the current eflicienc with which the arsenic is removed.

EXAMPLE III An arsenical electrolyte having the'analysis shown in Table 3 was electrolyzed, using a current of 4.6 amperes in the test cell. The results given in this table were obtained.

Table 3 Arsenic Removed H 304 AS C11 Ni Time, Hours gJL glllf Grams g./l. per Amp our Arsenic removal was slight until the dissolved copper 7 liter the electrolyte will contain more than about 4.5 grams I per liter of dissolved arsenic, by a process involving revention affords a method for the electrodeposition of armoval of copper and arsenic by electrolysis in separate steps, the improvement comprising subjecting the tankhouse electrolyte to electrolysis with an insoluble anode to remove copper from the electrolyte, discontinuing said electrolysis before the, resulting cathode becomes too soft for handling, thereafter concentrating the residual electrolyte by evaporation until its acid content is in the range of about 550-800 grams per liter, and electrolizing the concentrated electrolyte in said acid range with an insoluble anode until the arsenic content is reduced to an amount not below about 4.5 grams per liter.

2. A process for treating a sulfuric acid copper tankhouse electrolyte having nickel and arsenic dissolved therein, the arsenic being present in an amount such that when the electrolyte is concentrated by evaporation to an acid content of at'least about 550 grams per liter the electrolyte will contain more than about 4.5 grams per liter of 7 dissolved arsenic comprising electrolyzing said tankhouse electrolyte with an insoluble anode until the dissolved References Cited in the file of this patent UNITED STATES PATENTS 991,685 Aldrich et al May 9, 1911 1,148,522 Martin Aug. 3, 1915 OTHER REFERENCES Payne: Transactions of the American'Electrochemical Society, vol. 28, 1915, pages 111 to 116. 

1. IN THE REMOVAL OF ARSENIC FROM A SULFURIC ACID COPPER TANKHOUSE ELECTROLYTE HAVING A DISSOLVED ARSENIC CONTENT SUCH THAT WHEN THE ELECTROLYTE IS CONCENTRATED BY EVAPORATION TO AN ACID CONTENT OF AT LEAST ABOUT 550 GRAMS PER LITER THE ELECTROLYTE WILL CONTAIN MORE THAN ABOUT 4.5 GRAMS PER LITER OF DISSOLVED ARSENIC BY A PROCESS INVOLVING REMOVAL OF COPPER AND ARSENIC BY ELECTROLYSIS IN SEPARATE STEPS, THE IMPROVEMENT COMPRISING SUBJECTING THE TANKHOUSE ELECTROLYTE TO ELECTROLYSIS WITH AN INSOLUBLE ANODE TO REMOVE COPPER FROM THE ELECTROLYTE, DISCONTINUING SAID ELECTROLYSIS BEFORE THE RESULTING CATHODE BECOMES TOO SOFT FOR HANDLING, THEREAFTER CONCENTRATING THE RESIDUAL ELECTROLYTE BY EVAPORATION UNTIL ITS ACID CONTENT IS IN THE RANGEOF ABOUT 550-800 GRAMS PER LITER, AND ELECTROLIZING THE CONCENTRATED ELECTROLYTE IN SAID ACID RANGE WITH AN INSOLUBLE ANODE UNTIL THE ARSENIC CONTENT IS REDUCED TO AN AMOUNT NOT BELOW ABOUT 4.5 GRAMS PER LITER. 